Electrically conductive pastes composed of a base metal such as copper or nickel dispersed with an inorganic binder and an organic binder dispersed in a solvent are widely used as terminal electrode compositions for multilayer ceramic capacitors. These conductive paste compositions are typically fired in a neutral or reducing atmosphere (such as nitrogen) to suppress oxidation of the constituent metal and internal electrode. Accordingly, of the ingredients contained in the paste, it is necessary for the organic binder which must be burned off in the firing step to have sufficient thermal decomposability in this type of atmosphere. Various types of acrylic polymers are well suited for this purpose. For instance, JP-A 2001-307549 describes examples in which compounds such as methyl methacrylate, ethyl methacrylate and butyl methacrylate are used.
However, prior-art electrically conductive pastes have the following drawbacks. The fired film must have a thickness which is sufficiently low to satisfy the chip specifications and achieve good reliability during mounting. For example, at chip dimensions of 1.2 mm (width)×1.2 mm (thickness)×2.0 mm (length), a terminal film thickness of not more than 90 microns is appropriate. The method ordinarily used to control the film thickness within this range is a step called “blotting,” in which excess paste is removed after coating. However, this increases costs due to the increased manufacturing tact time it represents and the increase in man-hours required to recycle the paste that is removed.
Approaches that have been tried in order to eliminate blotting include that of optimizing the paste characteristics to enable the application of a thinner film. One such method has involved reducing the paste viscosity so as to lower the amount of paste deposited to the element assembly of the MLC. However, the low viscosity allows the paste to sag on the sidewalls, preventing the shape to be maintained.
Moreover, although film thickness-reducing effects can be expected with the use of a method for lowering the inorganic solids content within the paste, there is a corresponding increase in organic binder and thinner components, which lowers the viscosity. Increasing the amount of organic binder to check this decline in viscosity will ensure that a suitable viscosity is achieved, but an excessive increase in the amount of organic binder per unit volume harms thermal decomposability of the organic binder fired in a neutral or reducing atmosphere.
It is therefore an object of the invention to provide a thick film paste composition for terminal electrodes, which composition has a reduced solids content yet maintains a suitable paste viscosity and is able to ensure sufficient thermal decomposability of the organic binder.